Conventional semiconductor lasers rely upon the refractive index difference between the active layer of the material and the surrounding medium to provide an end-facet reflectivity which is high enough to sustain normal laser oscillation. Typically, the active layer material has an index of refraction of n=3.6 while the surrounding medium, usually air, has an index of refraction of 1.0. For most applications the differences between the refractive indexes has been sufficient to sustain the normal laser oscillation; however, the wide variety of uses that laser technology is expanding into have necessitated that a modification be made which enables satisfactory operation through different mediums.
Certain packaging constraints may require that the layer be immersed in a liquid or potted in a solid material. This would be the case where the laser needs protection from ambient influences or for ruggedization. Another application would be the immersion of the laser for underwater communications or a scientific application that places the end-facet in direct contact with the water transmitting medium. The water, dielectric oil or transparent potting materials usually have a relatively high refractive index, n, in the neighborhood of about 1.3. When conventional semiconductor lasers are so used, the end-facet reflectivity is decreased to a level which seriously alters the characteristics of the laser. The reduced facet reflectivity results in an increased threshold current, higher intensity noise and modal instability.
A few contemporary semiconductor laser designs have their end-facets coated with protective layers of aluminum oxide and silicon dioxide. In some lasers the coatings serve primarily to protect the end-facets from damage due to chemical attack. These protective coatings are typically an odd integral multiple of half-wavelength in thickness and therefore have no effect on the facet reflectivity which is determined solely by the refractive indexes of the active region and the surrounding medium.
The Multiple-Layer Reflector for Electroluminiscent Device by Michael Ettenberg in U.S. Pat. No. 4,092,659 concerns a reflector arrangement for optical radiation made of aluminum oxide, magnesium fluoride and silicon dioxide layers. The reflectivity of the layers on gallium arsenide and aluminum gallium arsenide materials of a semiconductor laser are variable to effect a desired percentage of reflectivity from nearly total reflectivity to a nearly minimal reflectivity. Ettenberg's modified device is said to have excellent reflecting properties and resistance to chemical attack during fabrication processing.
Applying layers of high index material across the facet surface of a semiconductor electroluminescent device was the subject matter of the U.S. Pat. No. 4,317,086 to Donald R. Scifres et al. Differing levels of reflectivity were provided for, it being found that a three layer reflector offered a relatively high modal reflectivity.
Thus, there is a continuing need in the state-of-the-art for an improvement for a semiconductor laser that modifies the end-facets to accommodate the presence of the liquid or solid immersion medium to restore the facet reflectivity to a more favorable level to sustain normal laser oscillation.